chapter7

Lectures Overview

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Chapter 7: Starlight and Atoms

Purpose of Chapter

• Presents astronomy as an organized understanding.

• Focuses on how the interaction of light with matter provides clues about celestial objects.

• Marks a transition to modern astrophysics, applying physics in the study of the sky.

Outline of Chapter

I. Starlight

• A. Temperature and Heat

• B. The Origin of Starlight

• C. Two Radiation LawsII. Atoms

• A. A Model Atom

• B. Different Kinds of Atoms

• C. Electron ShellsIII. Interaction of Light and Matter

• A. The Excitation of Atoms

• B. The Formation of a SpectrumIV. Stellar Spectra

• A. The Balmer Thermometer

• B. Spectral Classification

• C. The Composition of the Stars

• D. The Doppler Effect

• E. Doppler Shift Mechanism

• F. Calculating Doppler Velocity

• G. Shapes of Spectral Lines

Power of Starlight

• Analyzing light from stars provides insights on:

  1. Total energy output

  2. Surface temperature

  3. Radius

  4. Chemical composition

  5. Velocity relative to Earth

  6. Rotation period

Color and Temperature

• Stars exhibit different colors (blue, yellow, red) indicating temperature variations (e.g., Rigel, Sun, Betelgeuse).

Black Body Radiation

• Star light spectrum is approximately thermal, termed black body spectrum.

Laws of Black Body Radiation

1. Energy emitted increases with temperature: F = s*T^4

2. Peak wavelength shifts to shorter wavelengths as temperature increases (Wien’s Law).

The Color Index

• Color index is defined as B – V, comparing brightness in blue (B) and visual (V) bands.

• Smaller color index indicates hotter stars.

Light and Matter Interaction

• Spectra of stars show characteristic absorption lines, necessitating understanding atomic structure.

Atomic Structure

• Consists of a nucleus (protons and neutrons) and electrons. Nucleus contains the mass; the electron cloud occupies space.

Types of Atoms

• Determined by the number of protons.

• Most abundant elements: Hydrogen (1 proton), Helium (2 protons).

Electron Orbits

• Electrons occupy specific energy levels with unique energies for each element.

Atomic Transitions

• Electrons can transition to higher orbits by absorbing photons of precise energy.

Kirchhoff’s Laws of Radiation

1. Dense objects emit a continuous spectrum.

2. Low-density gas emits light at specific wavelengths (emission spectrum).

3. Continuous spectrum light through cool gas results in absorption spectrum.

Stellar Spectra

• Stars produce absorption spectra due to cooler surface layers absorbing specific frequencies.

Importance of Specific Absorption Lines

• Each element presents a unique set of lines to analyze cosmic compositions.

The Balmer Lines

• Specific hydrogen lines in the visible spectrum indicate transitions between energy levels.

Measuring Stellar Temperatures

• The strength of absorption lines can determine the temperature of stars.

Spectral Classification

• Different stars exhibit various absorption line patterns used for classification.

• Mnemonics assist in remembering star types.

The Doppler Effect

• Light from moving sources can exhibit blue (toward) or red (away) shifts, providing velocity information.

• Example: Balmer Alpha line shows shifting based on Earth’s motion.

Doppler Broadening

• Light absorption lines broaden due to random thermal motion of atoms, primarily influenced by Doppler effect.